Variable resistance unit



Mafh23,1943. y s, W. LAW 2,314,793

VARIABLE RESISTANCE UNIT Filed Aug. 5, 1940 A -z /74 lf3 $55.83

/Nl/EN-rofa STA/v1. EY l/V. LA W Patented Mar. 23, 1943 UNITED STATES PATENT 'OFFICE VARIABLE. RESISTANCE UNIT Stanley W. Law, St. Paul, Minn.

Application August 3, 1940, Seri-al No. 350,761

2 Claims.

Fone end of the usual conductor plate thereof and one of the terminal posts of the unit, which air gap is of a size to prevent the usual low frequency direct current used in operating a signal system, from jumping thereover, whereby such current must pass through the resistance coil of the unit. From actual experience in the field, I have found that frequently when the circuit of a signal system is struck'by lightning, the high voltage electrical discharge may not jump the air gap provided between the conductor plate andv one of the terminal posts, whereby the resistance coil may be severely damaged. To avoid this difficulty, I have found that certain materials have the inherent characteristic of present` ing a high resistance to the flow of low voltage current, such as used in the operation of an electrical signal system, and conversely, it presents a low resistance to the flow of high voltage electrical discharges, such as lightning, thereby increasing the safety and usefulness of a signaling system during times when lightning discharges are prevalent. Materials such as carbon and carborundum have been found very practical and ,efficientfor such purposes. By the employment of such materials in the construction of a resistance unit, I have found it possible to construct a lightningproof resistance unit of the character disclosed, in which a closed circuit is provided between the spaced terminal posts of the unit, through-which high voltage electrical discharges, such as lightning, may pass. Briefly, a block or piece of this material is interposed between the so-called free" end of the conductor plate of the resistance unit and adjacent terminal post, whereby the usual air-gap is eliminated, and at the same time both ends of the conductor plate may be firmly supported. By this novel arrangement, the electrical performance of the resistance unit is greatly improved in that the carbon or carborundum element positively prevents high voltage electrical discharges from passing through the resistance coil, such discharges passing directly through the conductor carbon element without in any way damaging the resistance coil.

An object of the present invention, therefore, is to provide a resistance unit of the character plate and disclosed wherein high voltage electrical discharges, such as lightning, may be short-circuited directly from one terminal post to the other of the resistance unit through a closed circuit, thereby avoiding damage to or burning out the usual resistance coil connected in the circuit.

Another important object of the invention is to provide a variable resistance unit of the class described, comprising a pair of terminal posts mounted in spaced relation on a suitable body of insulating material, and having a resistance coil in circuit therewith, and a conductor plate having one end secured to one of the terminal posts and having its opposite end spaced from the other of said posts, and an element having the inherent characteristic of presenting high resistance to low voltage current and low resistance to high voltage electric discharges, such, for example, as carbon, carborundum, and other similar Inaterials,. being interposed between the unsecured end of the conductor plate and the adjacent terminal post, whereby high voltage electrical discharges, such as lightning, may be short-circuited between the terminal posts without passing through the resistance coil, thereby protecting the coil from damage by high voltage electrical discharges. p

Other objects of the invention will appear from the following description and accompanying drawing and will be pointed out in the annexed claims.

In the accompanying drawing there has been disclosed a structure designed to carry out the various objects of the invention, but it is to be understood that the invention is not conned'to the exact features shown as various changes may be made within the scope of the claims which follow.

In the drawing:

Figure 1 is a longitudinal sectional View showing the general construction of my improved resistance unit;

Figure 2 is a plan view of Figure l;

Figure 3 is a crosssectional view on the line 3-3 of Figure 1;

Fig-ure 4 is a detail sectional view sho-wing the free end of vthe conductor plate supported by a single car-bon spacing element; and

Figure 5 is a similar view showing a construe--v tion wherein the carbon element is partially enclosed.

In the selected embodiment of the invention herein disclosed, there is illustrated in Figures 1 and 2, a variable resistance unit comprising a main supporting body, generally designated by the numeral 2, which may -be of porcelain or other insulating material applicable for the purpose. Suitable apertures 3 are shown provided at the ends of the body 2 whereby it may be secured to a suitable support, not shown.

Spaced terminal posts 4 and 5 are mounted in the insulating body 2, and are in circuitI with a resistance coil or wire 6, coiled about the body 2, as clearly illustrated in Figures 1 and 2. The insulating body is shown provided with grooves or depressions for receiving the convolutions 'I of the resistance wire, and whereby said convolutions are supported below the surface of the body and may be embedded in a suitable insulating material to insulate them against Contact with other metallic objects.

A conductor plate, generally designated by the numeral 8, has an aperture at one end traversed by the terminal post 5. Suitable nuts 9 are received in threaded engagement with the terminal post 5 between which the conductor plate 8 is firmly secured, as will be readily understood by reference to Figure 1. A conductor wire II is electrically connected to the terminal post 5 as shown.

The convolutions 'I of the resistance coil are adapted to be selectively connected in the circuit by means of a contact screw I2 mounted in threaded engagement with the conductor plate, as shown in Figure l. The conductor plate has a plurality of threaded apertures I3 each of which is disposed over one of the convolutions of the resistance coil, whereby the Contact screw may be selectively connected to said coils to vary the amount of resistance imposed in the circuit by the resistance coil, under normal operating conditions, as will readily be understood by reference to Figures 1 and 2. `To insure a good electrical connection between the contact screw and the selected convolution, each convolution is shown provided with a Contact area or button I4 adapted to be engaged by the pointed terminal of the contact screw.

The important feature of the present invention resides in the means provided for normally insulating the free end of the conductor plate 8 from the terminal post 4. As shown in Figure l, the conductor plate is .preferably provided with an enlarged opening or aperture I5, adapted to receive a reduced portion I6 of a suitable insulating element I'I of carbon, carborundum, orsome other similar material, mounted on the terminal post 4. A collar or annular element I8, of similar material, is mounted on the post beneath the element I'I, and its diameter is substantially the same as the outside diameter of said element. The elements II and I8 cooperate to support the adjacent end of the conductor plate therebetween, and the reduced portion I6 of the element I1 prevents the conductor plate from directly containing the post. A wire I9 is connected to the terminal post 4 and constitutes a portion of the circuit.

In the normal operation of the resistance unit. when used in an electrical signal system, the flow of current is through the wire II, terminal post 5, conductor plate 8, Contact screw I 2, to the convolution 'I of the resistance coil with which the contact screw is engaged, and thence through the convolutions of the resistance coil disposed between the contact screw I2 and the terminal post 4. to the wire I9. The contact screw I2 serves to vary the effective resistance in the circuit by increasing or decreasing the number of en'ective convolutions in the resistance coil, as is well known in devices of this type. Y

The insulating qualities of the elements I1 and I 3 are of such a nature that the normal low voltage direct current used in electrically operated signal systems, will not pass through the elements II and I8 whereby the normal ilow of current will be through the effective portion of the resistance coil, disposed between the contact screw I2 and terminal post 4.

If, however, the signal system is struck by lightning, the high voltage electrical discharge will 'pass directly through the elements I'I and I8 to the terminal post 4, such elements serving to provide in eifect a direct electrical connection between the conductor plate 8 and the terminal post 4 for such high electrical discharges, thereby protecting the resistance coil against damage by lightning or other high voltage electrical discharges.

In Figure 4 I have shown a construction of slightly difierent form wherein a single insulating element 2| is utilized to space the conductor plate l from the terminal post 4. In the form here shown the element 2I has a reduced portion received in the aperture Il of the conductor plate, which extends sufficiently below the bottom surface of the conductor plate to prevent said plate electrically contacting the terminal lpost 4.

In Figure 5 I have shown still another form wherein a single insulating element 22 is shown tted onto the terminal post 4 and is partially enclosed by suitable cup-shaped metallic elements 23. One of the elements 23 is seated directly on the conductor plate I and the other has a washer 24 seated thereon and clampingly engaged with a suitable nut 25. In the form shown in Figure 5 the openings provided in the cup-shaped elements 23 are relatively larger in diameter than the diameter of the terminal post 4, whereby the terminal post cannot electrically engage said cup-shapedv elements. It will also be noted by reference to Figure 5 that the porcelain body 2 is shown provided with a raised portion 26 adapted to be received in the aperture in the end of the conductor plate l thereby to prevent said plate from moving into contact with the terminal post 4. In the form shown in Figure 5 the ends of the resistance coil may be secured to the terminal posts 4 at the lower ends thereof, or within recesses 21 provided in the insulating body 2.

From the foregoing it will be noted that by the provision of the novel resistance unit herein disclosed, in a signal system, high voltage lighting discharges may pass freely through the conductor plate 4 and the carbon insulating block or element from one post to the other without any appreciable resistance, thereby minimizing the everpresent danger of the resistance coil becoming damaged by lightning. It will also be noted that in the resistance unit herein disclosed, the conductor plate 3 is firmly supported at each end, as distinguished from the structures disclosed in my above mentioned patents, in which only one end of the conductor plate is rmly supported. The carbon or carborundum elements I1 and I8 have the dual function of preventing low voltage direct current, which is normally used in the operation of electrical signal circuits, from short-circuiting between the terminal posts under normal operating conditions and, at the same time, the carbon blocks or elements provide a free passage for high voltage electrical discharges such as lightning, which is highly desirable in devices of this general type.

I have herein referred to the insulating elements I1 and I8 as being constructed of carbon or carborundum, but it is to be understood that they may be made of any suitable material applicable for the purpose, which has the inherent characteristics of presenting a high resistance path to the flow of low voltage current, such as commonly employed in the operation of an electrical signal system, and which will present a low resistance path to high voltage electrical discharges, such as lightning, whereby they provide, in effect, a closed circuit for such abnormal high voltage electrical discharges.

What I claim is:

1. A resistance unit adapted for use in a railroad signal control circuit, said unit comprising an insulating block, spaced terminal posts mounted in said block and xedly secured thereto, a resistance coil in direct circuit with said terminal posts, a conductor plate having one end xedly secured to one of said terminal posts and in circuit therewith, the intermediate portion of said conductor plate being disposed over the convolutions of said coil and spaced therefrom, a contact screw carried by the conductor plate for electrically connecting said plate to a selected convolution of said coil and whereby the effective resistance of said coil in the control circuit may be varied, a pair of carborundum elements mounted on the other of said terminal posts and between which the adjacent end of the conductor plate is supported, one of said elements having a reduced portion received in an aperture provided in the adjacent end of the conductor plate whereby said end of the plate is spaced from its respective terminal post, means on said terminal post for securing the carborundum elements in intimate contact with the conductor plate and whereby said plate is retained in fixed relation to the resistance coil, said carborundum elements presenting high resistance to direct current, such as used for operating railroad signal systems, whereby the normal flow of low voltage current in the control circuit will be through the resistance coil, and said elements presenting low resistance to high voltage surges such as lightning discharges, whereby when lightning strikes a rail or the control circuit, the high voltage current thereof will be shunted through the conductor plate and said carborundum elements, thereby preventing damage to the resistance coil.

2. A resistance unit adapted for use in a railroad signal control circuit, said unit comprising an insulating block, spaced terminal posts mounted in said block and iixedly secured thereto, a resistance coil in direct circuit with said terminal posts, a conductor plate having one end iixedly secured to one of said terminal posts and in circuit therewith, the intermediate portion of said conductor plate being disposed over the convolutions of said coil and spaced therefrom, a contact screw carried by the conductor plate for electrically connecting said plate to a selected convolution of said coil and whereby the effective resistance of said coil in the control circuit may be varied, a carborundum element mounted on the other of said terminal posts and supporting the adjacent end of the conductor plate, said element having a reduced portion received in an aperture provided in the adjacent end of the conductor plate whereby said end of the plate is spaced from lits respective terminal post, means for securing the carborundum element in intimate contact with the conductor plate whereby said plate is retained in xed relation to the resistance coil, said carborundum element presenting high resistance to direct current, such as used for operating railroad signal systems, whereby the normal fiow of low voltage current in the control circuit will be through the resistance coil, and said element presenting low resistance to high voltage surges such as lightning discharges, whereby when lightning strikes a rail or the control circuit, the high voltage current thereof will be shunted through the conductor plate and said carborundum element, thereby preventing damage to the resistance coil.

. STANLEY W. LAW. 

